Sovereign citizens have served fraudulent paperwork at banks and courthouses, the agent added.

"It's interesting to note that the homes that sovereign citizens chose to occupy are often high-dollar homes, often in the million-dollar range," said Emmett. "We find (it) very hypocritical that they seek out these nice homes in nice neighborhoods to occupy."

On March 3, a sheriff's department in an Atlanta suburb evicted a couple who had filed invalid papers claiming ownership and stayed in a foreclosed home for months, authorities said. Fearing the pair had guns, the department used a SWAT team.

The man was cooperative during the eviction, but had previously made threats to shoot anyone who came on his property, authorities said. "This threat was made to a representative of the mortgage company and reported to our office," said Major Jeff Mitchell of the Clayton County Sheriff's Department

The male resident, who claimed to be associated with the Black Hebrew Israelites, had first gone to court, arguing, "It would be unfortunate that the people are subject to Sheriff as the peace breaker and promoter of unhappiness, and the people have to protect themselves from the ones employed to protect the people," Mitchell said.

"We are working with the FBI to identify possible sovereign citizens living within Clayton County," Mitchell said.

The man was not arrested because he made no specific threat, according to CNN affiliate WSB. No weapons were found.

"We're definitely seeing an increase in the sovereign citizen movement," Sheriff Kemuel Kimbrough Sr. told WSB. "But this is the first one where we've had significant amount of threats of violence."

The FBI said sovereign citizens are responsible for many crimes, including impersonating police officers, threatening judges and using fake currency and documents.

The Southern Poverty Law Center in Montgomery, Alabama, has warned about the movement's growing popularity and calls it a magnet for far-right extremists.

"The sovereign movement is growing fast, and its partisans are clogging up the courts with their indecipherable filings," the center said in a report last year. "When cornered, many of them lash out in rage, frustration and, in the most extreme cases, acts of deadly violence."